Magnetically coupled flywheel

ABSTRACT

A stabilization system for a rotating load, such as a flywheel, includes a mechanical bearing to continuously support a shaft of the rotating load so as to hold the shaft at a substantially fixed axis of rotation. A magnetic stabilization assembly includes a plurality of electromagnets arranged around the shaft. Control circuitry for controls a resultant magnetic field generated by the electromagnets such that the magnetic field acts on a ferromagnetic element of the shaft to reduce imbalance forces acting on the shaft.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/353,064, filed on Mar. 14, 2019 and published asUS Patent Application Publication No. 2019/0252942 on Aug. 15, 2019,which is a continuation application of U.S. patent application Ser. No.15/592,215, filed on May 11, 2017 and published as US Patent ApplicationPublication No. 2017/0244300 on Aug. 24, 2017, which is a continuationapplication of U.S. patent application Ser. No. 14/418,114, filed onJan. 29, 2015 and published as US Patent Application Publication No.2015/0162799 on Jun. 11, 2015, now U.S. Pat. No. 9,667,117, which is aNational Phase Application of PCT International Application No.PCT/IL2013/050630, International Filing Date Jul. 24, 2013, which inturn claims the benefit of U.S. Provisional Patent Application No.61/677,056, filed on Jul. 30, 2012, of U.S. Provisional PatentApplication No. 61/722,174, filed on Nov. 4, 2012, and of U.S.Provisional Patent Application No. 61/773,158, filed on Mar. 6, 2013,all of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to flywheels. More particularly, thepresent invention relates to a magnetically coupled flywheel.

BACKGROUND OF THE INVENTION

Electrical energy storage systems are of use both to producers andconsumers of electrical energy. Storage systems for electrical energymay include storage batteries or other chemistry-based storage systems,capacitors or other electrically-based storage systems, thermal storage,or mechanical energy storage systems. Mechanical energy storage systemsmay include gravity-based storage systems, or inertial systems. Inertialsystems may include flywheel systems.

A typical flywheel system consists of a flywheel in the form of arotating mass that shares a common shaft with a rotor of amotor/generator unit. The rotating mass may include a material that issufficiently dense and strong to effectively store the energy whileremaining intact and operation. For example, the rotating mass mayinclude steel, a composite material, or a combination of such materials.The motor/generator unit functions as a motor during a charge phase ofthe system and as a generator during a discharge phase.

During a charge phase, electrical power to be stored by the flywheel isprovided to the motor/generator unit from a generating system or from anelectrical grid in the form of an electrical current. Themotor/generator unit then functions as an electrical motor. The currentcauses the rotor of the motor to generate a positive torque thatprovides angular acceleration to increase the rotational velocity, andthus the rotational kinetic energy, of the flywheel. The flywheelreaches a desired rotational velocity at which the flywheel is storing adesired quantity of energy. (Since rotational kinetic energy isproportional to the square of the angular velocity, flywheels aretypically designed to spin at high speed.) The charge phase then ends.

After the charge phase, a store phase (typically longer than the chargephase) may begin. During the store phase, the motor/generator unit maybe disconnected from any external electrical circuit, being thus placedin an idle mode. Thus, rotational inertia of the flywheel causes theflywheel to continue to rotate, storing the energy as rotational kineticenergy of the flywheel. During the store phase, various frictionalforces may act to slow the speed of rotation of the flywheel and tocause loss of the stored energy. Therefore, during the store phase,current may be provided intermittently and for brief periods to themotor/generator unit to restore lost energy.

When power is to be extracted from the flywheel system, a dischargephase is entered. During a discharge phase, energy that is stored in theflywheel is converted to electrical power and made available for use(e.g., via an electrical power grid). The motor/generator unit isconnected to an external electrical circuit and functions as agenerator. Rotation of the flywheel turns the rotor of the generator andgenerates electrical power while applying a decelerating or brakingforce to the flywheel. The discharge phase may continue until there isno longer a need for the stored energy. The flywheel system may thenrevert to the store phase. In other cases, the rotational velocity ofthe flywheel may be reduced during the discharge phase to less than aminimum velocity (e.g., below which the system is no longer capable ofgenerating usable electrical power). In this case the system may enter await phase.

During the wait phase, the flywheel may be stopped or may be rotating ata minimal velocity. The motor/generator is disconnected from externalcircuits and placed in an idle mode. The wait phase may continue untilelectrical energy is available to charge the flywheel again.

For an energy producer, energy storage enables provision of electricalpower to the electrical grid at a constant rate. For example, the rateof generation of electricity using renewable sources such as solar,wind, or tidal power may vary as the power source varies. Thus, at timeswhen electrical power production exceeds demand, excess produced energymay be stored. On the other hand, at times when demand for electricalpower exceeds production, the stored energy may be provided to theelectrical grid for use by consumers. Similarly, energy storage mayenable electrical power production at a constant rate, regardless ofmomentary demand. Thus, electrical power may be generated without a needfor (e.g., fuel based) generators that are operated only when demand ishigh (and may produce more carbon or pollutants than the generators thatare operated constantly).

Similarly, a storage system may be used by a consumer to save energycosts. For example, the cost of electrical power from the grid may varyperiodically. An electric power rate structure may charge more forelectrical power during peak demand hours and less during off-peak hours(e.g., a rate during peak hours may be triple the rate during off-peakhours). A consumer with an energy storage system may thus buy electricalpower during off-peak hours and use the saved energy during peak demandhours.

As compared with other energy storage techniques, systems, or methods, aflywheel provides some advantages. For example, the number ofcharge/discharge cycles is virtually unlimited, limited only by the wearof the mechanical parts. The amount and frequency of requiredmaintenance may thus also be low as compared with other systems.Flywheel systems may also be relatively insensitive to environmentalfactors such as temperature changes. A flywheel system does not requireuse of hazardous materials, does not emit harmful gasses, and componentsof the system may be recyclable at the end of the useful life of thesystem.

SUMMARY OF THE INVENTION

There is thus provided, in accordance with an embodiment of the presentinvention, a stabilization system for a rotating load, the systemincluding: a mechanical bearing to continuously support a shaft of therotating load so as to hold the shaft at a substantially fixed axis ofrotation; a magnetic stabilization assembly including a plurality ofelectromagnets arranged around the shaft; a control circuitry forcontrolling a resultant magnetic field generated by the electromagnetssuch that the magnetic field acts on a ferromagnetic element of theshaft to reduce imbalance forces acting on the shaft.

Furthermore, in accordance with some embodiments of the presentinvention, the ferromagnetic element includes a rotor ring.

Furthermore, in accordance with some embodiments of the presentinvention, the system includes a sensor to sense a vibration of theshaft, the control circuitry being configured to control the resultantmagnetic field so as to minimize the sensed vibration.

Furthermore, in accordance with some embodiments of the presentinvention, the control circuitry includes an H-bridge or a poweramplifier to drive the electromagnets to generate a desired magneticfield.

Furthermore, in accordance with some embodiments of the presentinvention, the control circuitry is configured to compensate for apreviously measured variation in a dimension of a mechanical component.

Furthermore, in accordance with some embodiments of the presentinvention, the rotating load includes a flywheel for storing energy.

Furthermore, in accordance with some embodiments of the presentinvention, the mechanical bearing includes a bearing selected from agroup of bearings consisting of a metal ball bearing, a hybrid ballbearing, and a ceramic ball bearing

There is further provided, in accordance with some embodiments of thepresent invention, a flywheel energy storage system including a flywheelwithin an evacuable enclosure, the flywheel including a core rotatableabout an axis of rotation and a plurality of rods, a proximal end ofeach rod being attached to a periphery of the core, the rods extendingsubstantially radially with respect to the axis of rotation.

Furthermore, in accordance with some embodiments of the presentinvention, the rods are attached to the periphery of the core in astaggered pattern.

Furthermore, in accordance with some embodiments of the presentinvention, each proximal end is attached to the periphery of the core byholder that is configured to hold the proximal end by a mechanismselected from a group of holding mechanisms consisting of a press fit, aself-locking wedge, high shear-stress glue, and a collapsible ferrule.

Furthermore, in accordance with some embodiments of the presentinvention, a distal end of a rod of said plurality of rods is weighted.

Furthermore, in accordance with some embodiments of the presentinvention, a rod of the plurality of rods includes fiberglass.

Furthermore, in accordance with some embodiments of the presentinvention, a rod of the plurality of rods includes a bundle of fiberswrapped around a column.

There is further provided, in accordance with some embodiments of thepresent invention, a flywheel energy storage system including: a DC bus;a plurality of flywheels; a plurality of motor/generator units, eachmotor/generator unit being rotatably coupled to a flywheel; a pluralityof controller/inverters, each controller/inverter being electricallycoupled to a motor/generator unit and to the DC bus; and a centralcontroller to control each controller/inverter so as to set a dischargerate for each of the flywheels when its motor/generator unit isoperating in a discharge mode, and to increase a voltage level of avoltage signal generated by the motor/generator unit in the dischargemode.

Furthermore, in accordance with some embodiments of the presentinvention, a controller/inverter includes an H-bridge circuit.

Furthermore, in accordance with some embodiments of the presentinvention, the central controller is configured to control acontroller/inverter of the plurality of controller/inverters to operatein a discharge mode while concurrently controlling anothercontroller/inverter of the plurality of controller/inverters to operatein a charge mode.

There is further provided, in accordance with some embodiments of thepresent invention, a flywheel energy storage system for storingelectrical energy, the system including: a flywheel with a rotatablemass and a shaft, the flywheel being enclosed within an evacuableenclosure, the shaft supported by bearings on opposite sides of therotatable mass; and an electric motor/generator unit having a stator anda rotor, the rotor being fixed to the shaft in a cantilevered mannerwithin the enclosure and being magnetically coupled to the stator, thestator being located outside of the enclosure.

Furthermore, in accordance with some embodiments of the presentinvention, a distance between the rotor and the stator is adjustable.

Furthermore, in accordance with some embodiments of the presentinvention, the stator is configured to couple to each of a plurality ofrotors.

Furthermore, in accordance with some embodiments of the presentinvention, the flywheel includes lead enveloped in a shell that includescarbon fiber.

Furthermore, in accordance with some embodiments of the presentinvention, the flywheel includes a plurality of glass fibers, each fiberbeing at least partially wrapped around a column of a plurality ofcolumns that are arranged in a circular pattern that is centered on anaxis of rotation of the flywheel, such that each fiber extendssubstantially radially outward from the axis when the flywheel rotates.

Furthermore, in accordance with some embodiments of the presentinvention, the flywheel includes a structure with an eccentric massdistribution that is rotatable to adjust a balance of the flywheel.

Furthermore, in accordance with some embodiments of the presentinvention, section of the enclosure between the rotor and the statorincludes glass.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the present invention, and appreciate itspractical applications, the following Figures are provided andreferenced hereafter. It should be noted that the Figures are given asexamples only and in no way limit the scope of the invention. Likecomponents are denoted by like reference numerals.

FIG. 1 schematically illustrates a flywheel energy storage system with aflywheel magnetically coupled to a rotor of a motor/generator unit, inaccordance with an embodiment of the present invention.

FIG. 2 schematically illustrates sharing of a single motor/generatorunit by a plurality of flywheel units of a flywheel energy storagesystem, in accordance with an embodiment of the present invention.

FIG. 3 schematically illustrates a flywheel energy storage system withmagnetic reduction transmission, in accordance with an embodiment of thepresent invention.

FIG. 4 schematically illustrates a flywheel with a shell construction,in accordance with an embodiment of the present invention.

FIG. 5A schematically illustrates a brush-like flywheel that includesradially projecting rods, in accordance with an embodiment of thepresent invention.

FIG. 5B schematically illustrates a staggered arrangement of rods of thebrush-like flywheel shown in FIG. 5A.

FIG. 6 schematically illustrates operation of a rod holder of thebrush-like flywheel shown in FIG. 5A.

FIG. 7 schematically illustrates operation of a collapsible ferule tohold a projecting rod.

FIG. 8 schematically illustrates a variant of the brush-like flywheelshown in FIG. 5A, in which the radially projecting rods are weighted.

FIG. 9 schematically illustrates structure of a cassette of a glassfiber brush-like flywheel, in accordance with an embodiment of thepresent invention.

FIG. 10 illustrates a technique to form a fiber bundle for the cassetteshown in FIG. 9.

FIG. 11 schematically illustrates an open-frame motor/generator unit, inaccordance with an embodiment of the present invention.

FIG. 12 schematically illustrates a flywheel cluster, in accordance withan embodiment of the present invention.

FIG. 13A schematically illustrates an active magnetic balancing systemfor a flywheel of a flywheel energy storage system, in accordance withan embodiment of the present invention.

FIG. 13B shows a top view of the active magnetic balancing system shownin FIG. 13A.

FIG. 13C schematically illustrates control of the active magneticbalancing system shown in FIG. 13A using a three-phase H-bridge.

FIG. 13D schematically illustrates control of the active magneticbalancing system shown in FIG. 13A using power amplifiers.

FIG. 14A is a schematic illustration of a flywheel balancing assemblyfor a flywheel energy storage system, in accordance with an embodimentof the present invention.

FIG. 14B is a side view of the flywheel balancing assembly shown in FIG.14A.

FIG. 15 schematically illustrates a flywheel energy storage system thatincludes an array of flywheel units, in accordance with an embodiment ofthe present invention.

FIG. 16 schematically illustrates a flywheel energy storage system thatis directly connected to a renewable energy DC bus, in accordance withan embodiment of the present invention.

FIG. 17 schematically illustrates a flywheel energy storage system thatincludes a constant voltage DC bus, in accordance with an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those of ordinary skill in the artthat the invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components, modules,units and/or circuits have not been described in detail so as not toobscure the invention.

Embodiments of the invention may include an article such as a computeror processor readable medium, or a computer or processor storage medium,such as for example a memory, a disk drive, or a USB flash memory,encoding, including or storing instructions, e.g., computer-executableinstructions, which when executed by a processor or controller, carryout methods disclosed herein.

In accordance with embodiments of the present invention, a flywheelenergy storage system includes a flywheel. The flywheel, as well as anyshaft, axle, or other rotating component that is rotatable together withthe flywheel, is enclosed within a vacuum enclosure. Air, or othergaseous or fluid contents of the vacuum enclosure, may be evacuated toform a vacuum within the vacuum enclosure.

Enclosing the flywheel within vacuum that is formed within an evacuatedvacuum chamber may be advantageous. Operating the flywheel in a vacuumminimizes air drag on rotating components of the flywheel.

In accordance with embodiments of the present invention, some or allcomponents of a motor/generator unit of the flywheel energy storagesystem are located outside of the vacuum enclosure. For example, theentire motor/generator unit may be located outside of the vacuumenclosure. As another example, a stator of the motor/generator unit maybe located outside of the vacuum enclosure, while a rotor of themotor/generator unit is located within the vacuum enclosure.

Placement of the motor/generator unit outside of the vacuum enclosuremay be advantageous as compared with a system in which themotor/generator unit is also enclosed within a vacuum enclosure. Forexample, placement of the motor/generator unit outside of the vacuum mayenable a motor/generator unit to be movable among an array of enclosedflywheels. Thus, the movable motor/generator unit may be coupled atdifferent times to different flywheels in the array. In this manner, byreducing the number of required motor/generator units, cost of amultiple flywheel system may be reduced (relative to a system thatrequires a dedicated motor/generator unit for each flywheel).Furthermore, placement of the motor/generator unit outside of the vacuumenclosure may simplify cooling of the motor/generator unit (e.g., byenabling convection or conductive cooling).

In accordance with embodiments of the present invention, the flywheel iscoupled to the motor/generator unit via magnetic coupling. Magneticcoupling of the flywheel to the motor/generator unit may beadvantageous. Magnetic coupling eliminates any requirement for vacuumsealing about the flywheel shaft that could introduce mechanicalfriction on the shaft.

Magnetic coupling may include coupling a rotating first magnetic elementof the flywheel that rotates together with the flywheel to a secondmagnetic element that rotates together with the rotor of themotor/generator unit. In this case, the rotor of the motor/generatorunit is located outside of the vacuum enclosure. Magnetic coupling inthis case may enable the motor to stop rotating when in an idle mode.For example, the first and second magnetic elements may be moved apartto decouple the rotations. This may reduce wear of the rotatingcomponents and bearings of the motor/generator unit, as well as reducinglosses caused by eddy currents and by hysteresis within the iron core ofthe stator.

Magnetic coupling may include an electromagnetic interaction between arotor of the motor/generator unit that rotates together with theflywheel (e.g., at the end of a shaft of the flywheel inside the vacuumenclosure) with a stator of the motor/generator unit that is locatedoutside of the vacuum enclosure.

The mass of the flywheel may be greater than 50 kg. More specifically,the mass may be in the range of 100 kg to 200 kg.

FIG. 1 schematically illustrates a flywheel energy storage system with aflywheel magnetically coupled to a rotor of a motor/generator unit, inaccordance with an embodiment of the present invention.

Flywheel energy storage system 90 includes a flywheel unit 100 and amotor/generator unit 200.

Flywheel unit 100 includes flywheel 110. Flywheel 110 includes a massthat is rotatable about flywheel shaft 140. Flywheel shaft 140 issupported by bearings 120. For example, bearings 120 may include ballbearings made, e.g., of steel or of ceramic materials, magneticbearings, a combination of magnetic bearings and ball bearings, oranother type of bearing. Flywheel shaft 140 and flywheel 110 arerotatable at a high angular velocity within vacuum enclosure 150.

Flywheel shaft 140 is provided with flywheel magnetic coupling plate 130at an end of flywheel shaft 140 that is proximal to motor/generator unit200. Flywheel magnetic coupling plate 130 is mounted in a cantileveredmanner to flywheel shaft 140. As used herein, cantilevered attachment ormounting of an object (e.g., a coupling plate or rotor) to a shaftindicates that the object is supported only by the shaft, without anysupport on the side of the object opposite the side from which the shaftextends. Flywheel magnetic coupling plate 130 may include one or moremagnets, or a ferromagnetic material that is attracted to a magnet onmotor/generator magnetic coupling plate 210 of motor/generator unit 200.The magnets may include permanent magnets or electromagnets. The magnetsmay be embedded or enclosed within flywheel magnetic coupling plate 130or may be mounted to a surface of flywheel magnetic coupling plate 130.Alternatively or in addition to being mounted at an end of flywheelshaft 140, a magnetic coupling plate may be mounted directly to flywheel110 or to another component of flywheel unit 100 that rotates with thesame rotational velocity as flywheel 110.

Flywheel magnetic coupling plate 130 may be operated to function as partof a magnetic coupling to motor/generator unit 200. For example,flywheel magnetic coupling plate 130 may be coupled to motor/generatormagnetic coupling plate 210 of motor/generator unit 200. Motor/generatormagnetic coupling plate 210 may include one or more magnets, or aferromagnetic material that is attracted to a magnet on flywheelmagnetic coupling plate 130. Motor/generator magnetic coupling plate 210may be configured to be rotatable with a rotor of motor/generator 220.Flywheel magnetic coupling plate 130 may be positioned close to (e.g.,within a few tenths of a millimeter of) coupling cover 160 of vacuumenclosure 150. Similarly, motor/generator magnetic coupling plate 210may be positioned close to (e.g., within a few tenths of a millimeterof) coupling cover 230 of motor/generator unit enclosure 250.High-torque magnetic coupling devices are known in the art and arecommercially available.

All or part of coupling covers 160 and 230 may be constructed of amagnetically susceptible material that enables magnetic coupling betweenflywheel magnetic coupling plate 130 and motor/generator magneticcoupling plate 210. For example, coupling cover 160 or 230 may includealuminum, glass, plastic, or another magnetically susceptible material.

Motor/generator magnetic coupling plate 210 may be connected to shaft215 of motor/generator unit 200. Shaft 215 may be connected (directly orindirectly, e.g., via a transmission 240) to a rotor of motor/generator220.

In accordance with some embodiments of the present invention,motor/generator unit 200 may be enclosed within motor/generatorenclosure 250. Motor/generator enclosure 250 may be sealed so as toisolate motor/generator 220 from the ambient environment. In this case,motor/generator enclosure 250 may be evacuated or filled with a rarifiedgas (e.g., air).

In accordance with other embodiments of the present invention,motor/generator 220 and motor/generator magnetic coupling plate 210 (andtransmission 240) may be open to the ambient atmosphere. Exposingmotor/generator 220 to the atmosphere may enable use of standardcomponents in motor/generator unit 200. Furthermore, Exposingmotor/generator 220 to the atmosphere may simplify cooling ofmotor/generator 220 (e.g., using standard air or liquid convection orconduction techniques) and may eliminate any need for handlingoutgassing of components as could be required in a sealed enclosure.

Flywheel energy storage system 90 may be provided with a mechanism toenable gradual engagement or disengagement of flywheel magnetic couplingplate 130 with motor/generator magnetic coupling plate 210. For example,the gradual engagement/disengagement mechanism, as represented byengagement/disengagement movement 260, may enable movement ofmotor/generator magnetic coupling plate 210 toward or away from flywheelmagnetic coupling plate 130. The engagement/disengagement mechanism mayalign the axis of rotation of motor/generator magnetic coupling plate210 with the axis of rotation of flywheel magnetic coupling plate 130while a separation distance between the plates is too large to enablemagnetic coupling (e.g., during a store or wait phase).Engagement/disengagement movement 260 may be applied to graduallyshorten the separation distance. As the distance is reduced slowly,motor/generator magnetic coupling plate 210 gradually engages flywheelmagnetic coupling plate 130 while one of the plates is rotating at highspeed. For example, a speed of rotation of a rotor of motor/generator220 may be controlled to match a rotational speed of flywheel magneticcoupling plate 130 during, or prior to, engagement.

As another example, at the start of a charge phase, motor/generatormagnetic coupling plate 210 may be rotating faster than flywheelmagnetic coupling plate 130. On the other hand, at the start of adischarge phase, flywheel magnetic coupling plate 130 may be rotatingfaster than motor/generator magnetic coupling plate 210. Gradualengagement causes the rotational velocity of the more slowly rotatingplate to gradually increase until the rotational velocities of the twoplates are approximately equal.

Alternatively or in addition, the rotor of motor/generator 220 (andmotor/generator magnetic coupling plate 210) may be angularlyaccelerated (e.g., by motor operation of motor/generator 220) to arotation velocity that is close to that of flywheel magnetic couplingplate 130. Engagement/disengagement movement 260 may then be operated toreduce the separation distance between motor/generator magnetic couplingplate 210 and flywheel magnetic coupling plate 130 until the platesengage one another.

Placement of motor/generator unit 200 outside of flywheel vacuumenclosure 150 may enable use of different motor/generator units 200 witha single flywheel unit 100. For example, different motor/generator unitsmay be distinguished from one another by different gear ratios (e.g., ahigh ratio reduction gear for high speed operation of flywheel 110, anda low reduction ratio or no gear at all for lower speed operation offlywheel 110).

Placement of the motor/generator unit outside of flywheel vacuumenclosure 150 may enable use of separate motors or generators in placeof a single device with both motor and generator capabilities. Eachseparate motor or generator device may be used during the appropriatephase of operation of a flywheel energy storage system. Such separatemotors and generators may be less expensive and simpler than a singledevice with capabilities of both.

In accordance with some embodiments of the present invention, flywheelunit 100 may include magnetic thrust bearings. The magnetic thrustbearings are configured to counter any axial forces that are applied toflywheel shaft 140 during operation of flywheel energy storage system90. For example, the magnetic thrust bearings may operate on magneticplate 170. Magnetic plate 170 is attached to an end of flywheel shaft140 that is distal to motor/generator unit 200. Magnetic plate 170 maybe permanently magnetized (e.g., is made of or includes soft iron oranother ferromagnetic material). Magnetic plate 170 may rotates at thesame high rotational velocity as flywheel 110. Therefore, the permanentmagnets of magnetic plate 170 may be encapsulated so as to preventdisintegration or rupture of magnetic plate 170 when spinning at highspeed. The encapsulation may be made from carbon fiber composite or fromnonmagnetic metal.

The magnetic thrust bearings may include permanent magnets 181 that arelocated on rear plate 165 of flywheel vacuum enclosure 150. Permanentmagnets 181 may include ring magnets that are arranged concentricallyabout the longitudinal axis of flywheel shaft 140, or block orcylindrical magnets that are arranged in a circular pattern.

Permanent magnets 181 are configured to repel the magnets on magneticplate 170. Thus, when magnetic plate 170 is placed at the bottom end ofa vertically oriented flywheel shaft 140, permanent magnets 181 andmagnetic plate 170 cooperate to at least partially support the weight offlywheel 110 (and thus reduce stress on bearings 120). Similarly, in aninverted system (e.g., where gravity may tend to increase the distancebetween magnetic plate 170 and permanent magnets 181), permanent magnets181 may be configured to attract the magnets (or a ferromagneticmaterial) on magnetic plate 170.

In accordance with some embodiments of the present invention, adjustmentdevice 180 may be provided to enable fine adjustment of the equilibriumposition of flywheel shaft 140. For example, such fine adjustment mayenable compensation for changes in the effective weight of flywheel 110with regard to bearings 120. For example, when motor/generator magneticcoupling plate 210 engages flywheel magnetic coupling plate 130, anaxial force may be exerted on flywheel shaft 140. Adjustment device 180may include a magnet (e.g., permanent magnet or electromagnet). Forexample, the magnet of adjustment device may be configured to repel orattract magnetic plate 170 (depending on the configuration of flywheelenergy storage system 90). Adjustment movement 190 may be applied toadjustment device 180 to adjust the force that is exerted on magneticplate 170 so as to counteract a force that is exerted by motor/generatormagnetic coupling plate 210 on flywheel magnetic coupling plate 130.Adjustment movement 190 and engagement/disengagement movement 260 may becontrolled by single controller that is configured to coordinate themovements with one another (and thus maintain a constant axial force onflywheel shaft 140).

In accordance with some embodiments of the present invention, a flywheelenergy storage system may include a plurality of flywheel units, eachenclosed in a separate vacuum enclosure. Placement of themotor/generator unit outside of flywheel vacuum enclosure 150 may enablemoving a single motor/generator unit 200 from one flywheel unit toanother. Thus a single motor/generator unit 200 may be shared by severalof the flywheel units. The cost of such a system may thus be reduced.

For example, a flywheel unit with a steel flywheel rotor, e.g., with amass of about 400 kg and rotating at a speed of about 15,000 revolutionsper minute (rpm) may store about one kilowatt-hour (kWh) of energy. Ifgreater energy-storage capacity is required, then multiple flywheelunits may be provided.

FIG. 2 schematically illustrates sharing of a single motor/generatorunit by a plurality of flywheel units of a flywheel energy storagesystem, in accordance with an embodiment of the present invention.

Multiple flywheel energy storage system 300 includes a plurality offlywheel units 101 a-101 c, shown as arranged in a single row (more thanone row of flywheel units may be included). Each row of flywheel units101 a-101 c is provided with a single motor/generator system 310,controlled by system controller 325. Controller 325 is configured tooperate in accordance with programmed instructions.

For example, controller 325 may be configured to wait for a command froma main controller. When the command is received, controller 325 causesmovement control system 315 to connect the motor/generator unit 200 toflywheel unit 101 a. An indication may be received that motor/generatorunit 200 is correctly placed and has engaged successfully flywheel unit101 a. Controller 325 then operates motor/generator unit 200 in a motormode to accelerate the flywheel of flywheel unit 101 a. Controller 325monitors the speed of one or both of the motor/generator and theflywheel. When the monitored speed is within a pre-determined thresholdof the target speed, power to motor/generator unit 200 is discontinued.Controller 325 the causes movement control system 315 to disengage themotor/generator unit 200 from flywheel unit 101 a.

Once movement control system 315 indicates successful disengagement, andif more of flywheel units 101 b or 101 c are to be energized, e.g.,flywheel unit 101 c, controller 325 may cause movement control system315 to move motor/generator unit 200 to flywheel unit 101 c (asindicated by motor/generator unit 200′). The preceding process is thenrepeated until all flywheel units are spinning at a target angularvelocity (and multiple flywheel energy storage system 300 is storingquantity of energy equal to its full capacity), or power is to beextracted from the system.

When the energy stored in multiple flywheel energy storage system 300 isapproximately equal to the system's capacity, motor/generator unit 200may be disengaged from all of flywheel units 101 a-101 c.Motor/generator unit 200 may be positioned at a standby location.Alternatively or in addition, motor/generator unit 200 may remain nearone of flywheel units 101 a-101 c but in an idle mode. Thus, if power isto be provided from the flywheel, motor/generator unit 200 may be placedin a generator mode, thus providing uninterrupted power supply (UPS)functionality.

When power is to be extracted from the multiple flywheel energy storagesystem 300 array, controller 325 may cause movement control system 315to position a motor/generator unit 200 to engage one of flywheel units101 a-101 c (e.g., 101 a). Motor/generator unit 200 engages the flywheelunit (e.g., 101 a) and operates in a generator mode.

When engaging the flywheel unit (e.g., 101 a), a rotor ofmotor/generator unit 200 accelerates to the speed of the flywheel of theflywheel unit (e.g., 101 a). Electrical power is then generated. Theelectrical power varies in frequency and amplitude as the rotation ofthe flywheel decelerates. Thus, signal conditioning may be providedprior to feeding the generated power into a power grid. For example,motor/generator unit 200 may be associated with power converter 320.Power converter 320 may be incorporated into, mounted on, or locatednear motor/generator unit 200.

The output signal of power converter 320 may be at a level that enablesthe signal to be connected to a common power bus. The common power busmay aggregate electrical power that is generated by a plurality ofmotor/generator units of a multiple flywheel energy storage system 300.

In accordance with another embodiment of the present invention, separatemotor and generator units may be provided. The motor or generator unitmay be moved separately throughout an array of flywheel units by amovement control system.

In accordance with another embodiment of the present invention, amotor/generator unit may be provided with a variable transmission (e.g.,a gear box). The variable transmission may be controlled to reduce orincrease a difference in rotational velocity between the motor/generatorunit and the flywheel of a flywheel unit. This may enable use ofhigh-efficiency or low-cost commercially available devices that are notcapable of operating at the high speed at which the flywheel isrotating.

In accordance with another embodiment of the present invention, flywheelunits are arranged in two or more arrays. This may allow for continuousprovision of energy during operation of a multiple flywheel energystorage system. For example, the motor/generator unit of one array maybe engaged and operating while the motor/generator unit of the otherarray is disengages. Alternatively or in addition, the system can beconstructed that at least two motor/generator units are movablethroughout a single array of flywheels units. Flywheel modules may bearranged in a linear, circular, curved, or other arrangement, or in theform of a two- or three-dimensional matrix.

In accordance with an embodiment of the present invention, atransmission (e.g., transmission 240 as shown in FIG. 1) may be utilizedto enable the flywheel to rotate at a higher angular velocity andexerting a low torque to engage a motor/generator unit whose rotor isrotating at a lower angular velocity and with a large torque. Forexample, a rotor of a motor/generator unit may rotate at 6,000 rpm whileengaging a flywheel that is rotating at 60,000 rpm or more. Suchtransmissions are known in the art and are commercially available. Thetransmission may include mechanical components (e.g., gears) or may bebased on magnetic interactions.

In accordance with an embodiment of the present invention, variousmotor/generator units of a flywheel energy storage system may beprovided with different transmissions 240. Each transmission 240 mayprovide a different transmission ratio. For example, a motor/generatorunit having a transmission 240 that provides a transmission ratio of 1:3could be used when the angular velocity of the flywheel is less thanabout 15,000 rpm. Another motor/generator unit may include atransmission 240 that provides a transmission ration of 1:6 may be usedfor flywheel rotations for up to 60,000 rpm or more. Other transmissionratios may be used.

In accordance with some embodiments of the present invention, a magneticcoupling between a flywheel unit and a motor/generator unit may providefunction of a reduction gear or transmission.

FIG. 3 schematically illustrates a flywheel energy storage system withmagnetic reduction transmission, in accordance with an embodiment of thepresent invention.

Magnetic pinion 410 is mounted on flywheel shaft 430 of flywheel unit102. Thus, magnetic pinion 410 rotates together with flywheel 110. Cap162 of vacuum enclosure 152 is placed near (e.g., within a few tenths ofa millimeter) magnetic pinion 410 and is made of a magneticallysusceptible material. Thus, magnetic pinion 410 may be magneticallycoupled laterally to magnetic gear 420 of motor/generator unit 202.Magnetic gear 420 may be connected, via shaft 215 to a rotor ofmotor/generator unit 202. Engagement/disengagement movement 260 may beoperated to cause magnetic gear 420 to magnetically engage magneticpinion 410.

Magnetic gear 420 may typically have a larger radius than magneticpinion 410. Thus, when magnetically coupled with one another, flywheel110 may rotate faster than the rotor of motor/generator unit 202. Thus,the magnetic coupling between magnetic gear 420 and magnetic pinion 410may provide the function of a reduction gear.

Other configurations that provide both magnetic coupling and reductiontransmission are possible. For example, the magnetic gear of themotor/generator unit may have an annular configuration within whichmagnetic pinion 410 may rotate.

The maximum angular velocity at which a flywheel may safely spin maydepend on the material from which the flywheel is constructed (e.g., thetensile strength of the material) and its structure. For example, aflywheel that is constructed from steel may be limited to a maximumangular velocity of about 20,000 rpm. A flywheel rotor may beconstructed using composite materials (e.g., having a lower density thansteel but a much higher tensile strength). A flywheel constructed usinga composite material may have a maximum angular velocity of about 50,000rpm or more.

Carbon fibers having high tensile strength have been used to form aflywheel rotor assembly. Such flywheels have been known to fail by alaminate disintegration mechanism due to the low sheer and tensilestrength of an epoxy adhesive matrix used to bond layers of the carbonfibers.

In accordance with an embodiment of the present invention, a flywheelrotor may be constructed using carbon fibers in which carbon nano-tubesare added to an adhesive epoxy resin. The nano-tubes may increase theinterlaminate strength of the composite material, and may thus enablethe rotor to operate at high rotational velocities withoutdisintegration.

In accordance with an embodiment of the present invention, a flywheelrotor may be constructed with a low-density outer shell with hightensile strength that envelopes a high-density inner core having lowertensile strength. Such a flywheel may be rapidly rotated without causingdisintegration of the high-density material.

FIG. 4 schematically illustrates a flywheel with a shell construction,in accordance with an embodiment of the present invention.

Flywheel 112 includes an outer shell 510 that envelopes an inner shell520. Outer shell 510 and inner shell 520 surround inner core 530.

Outer shell 510 is constructed to have a high tensile strength. Forexample, outer shell 510 may include a carbon fiber composite materialhaving a high tensile strength by inclusion of carbon nano-tubes in theepoxy resin used to bond the material. Thus, outer shell 510 maywithstand the very high tensile stress caused by high speed rotation offlywheel 112.

Inner shell 520 may include a high density material. The high densitymaterial in inner shell 520 may be included to increase the moment ofinertial of flywheel 512.

For example, high density material in inner shell 520 may include lead.Lead may be cast using centrifugal casting into outer shell 510 whichserves as a cast mold. (As opposed to traditional centrifugal casting,the cast mold is not removed after casting.) In this manner, lead, whichexhibits a relatively low tensile strength but has a high density, maybe utilized to increase the mass (and thus the moment of inertia) ofinner shell 520 and of flywheel 112. The composite material insurrounding outer shell 510 prevents the lead in inner shell 520 fromdisintegrating during high-speed rotation of flywheel 112. Othercombinations of materials may be used.

Due to the high cost of carbon fiber, a flywheel may be designed tolimit the quantity of high tensile strength material that isincorporated into the flywheel while providing a sufficiently largemoment of inertia. The design may also reduce the risk of catastrophicfailure of the flywheel during high-speed rotation. In accordance withan embodiment of the present invention, the flywheel may includeradially projecting rods.

FIG. 5A schematically illustrates a brush-like flywheel that includesradially projecting rods, in accordance with an embodiment of thepresent invention.

Brush-like flywheel 550 includes flywheel core 551 from whose peripheryprojecting rods 554 extend radially. (Extending from the periphery isused herein to exclude a configuration in which both ends of a singlerod that is inserted into a core extend outward on different sides ofthe core.) For example, flywheel core 551 may be made of steel oranother dense material.

Projecting rods 554 may include fiberglass. Fiberglass, althoughexhibiting high tensile strength (S-glass has a specific strength thatis greater than that of carbon fiber), is deformable (exhibiting largestrain when subjected to a stretching force, being characterized by alow specific modulus relative to carbon fiber and many other materials).Therefore, dimensions of an enclosure that encloses brush-like flywheel550 may be sufficiently large to accommodate stretching of projectingrods 554.

For example, the rod may be subjected to a pultrusion process in whichthe glass fiber bundles are immersed in a matrix material and pulledthrough a heated die. The glass fibers may include a large number ofmicro-fibers, for example, each having a diameter in a range of 7 μm to20 μm. This pultrusion process creates a high-density high-strength rodthat has an axial structure (all fiber bundles being aligned essentiallyparallel to the rod axis). The matrix material is of a thermosettingnature. Thus, the matrix material is cured during its passage throughthe heated die. The rigidity of the resulting rods is substantiallyincreased by the pultrusion process. Thus, the pultrusion processing mayenable use of relatively low cost materials, such as S-glass or E-glass,that could not be used otherwise.

Interlayer shear strength of the rod construction may be increased bythe addition of carbon nano-tubes into the matrix material. An optimaltype and quantity of nano-tubes to be added may be determined bytesting. For example, carbon nano-tubes may make up 0.03% by weight ofthe matrix material. In some cases, measured rod strength has been foundto increase by 20%-30% as a result of addition of carbon nano-tubes.

For example, projecting rods 554 made out of fiberglass may each have adiameter as large as about 100 mm, or a typical value of about 12 mm, oranother diameter.

Projecting rods 554 may be attached to the periphery of the flywheelcore in a staggered pattern or arrangement. A staggered arrangement mayincrease uniformity of stress distribution and reduce maximal stress inflywheel core 551.

FIG. 5B schematically illustrates a staggered arrangement of rods of thebrush-like flywheel shown in FIG. 5A. In the staggered arrangementshown, projecting rods 554 (viewed head on) that extend from theperiphery of flywheel core 551 are arranged in rows 553 a-553 c. Rows553 a-553 c are staggered with respect to one another such that, forexample, row 553 b is shown as laterally displaced with respect to row553 a and 553 b. Other staggered arrangements of projecting rods 554 onflywheel core 551 are possible.

Brush-like flywheel 550 is operated in an evacuated vacuum enclosure(e.g., flywheel vacuum enclosure 150 as shown in FIG. 1). Operation inan evacuated enclosure eliminates or reduces aerodynamic drag onprojecting rods 554. During rotation of brush-like flywheel 550 aboutits axis, projecting rods 554 are subjected to uniaxial loading due tothe centrifugal force and their length increases. For example, eachprojecting rod 554 may extend by 1-2% of its length (such that eachprojecting rod 554 is about half a meter long, the diameter ofbrush-like flywheel 550 may increase by one or two centimeters) whenrotating at full speed. Therefore, the vacuum enclosure is designed withsufficient diameter to prevent the distal tips of projecting rods 554from coming into physical contact with the enclosure.

Projecting rods 554 are connected to the periphery of core 551 by rodholders 552. For example, a proximal end of rod holder 552 may beattached (e.g., screwed or glued into, or otherwise secured) to core551, e.g., into a tapped hole on core 551 (the tapped hole and rodholder 552 are typically designed to withstand centrifugal forces on aprojecting rod 554 held by a rod holder 552). A proximal end of each rodholder 552 may be screwed into a tapped hole in core 551, or otherwisesecured to core 551, after a projecting rod 554 is inserted into, andheld by, a distal end of rod holder 552. Alternatively or in addition, arod holding structure may be incorporated into core 551.

FIG. 6 schematically illustrates operation of a rod holder of thebrush-like flywheel shown in FIG. 5A.

In accordance with some embodiments of the present invention, aprojecting rod 554 may be attached to a rod holder 552 by press fit. Inthis case, a proximal end (or all of) projecting rod 554: is cooled(e.g., by liquid nitrogen or another coolant) so as to reduce itsdiameter. The cooled end is inserted into an accurately machined cavityof rod holder 552. When the end of projecting rod 554 warms, theproximal end of projecting rod 554 expands and fills the cavity.Projecting rod 554 is thereafter held in base by friction with the wallsof the cavity.

Alternatively or in addition, a cavity 558 of rod holder 552 includeswedge-shaped spaces, as shown in FIG. 6. The spaces may be filled withglue, such as a high shear-stress glue, or a matrix material as aproximal end of projecting rod 554 is inserted into cavity 558. When thematerial hardens, the material and cavity 558 serve as a self-lockingwedge mechanism that prevents the projecting rod 554 from being pulledout of cavity 558 by centrifugal forces.

Alternatively or in addition, a projecting rod 554 may be held to acavity in a rod holder 552 or in core 551 by a collapsible ferule.

FIG. 7 schematically illustrates operation of a collapsible ferule tohold a projecting rod.

Collapsible ferule 560 includes a machined (or otherwise formed) metalpart with a conical external shape. The internal shape of collapsibleferule 560 is shaped to a profile that is designed to create a desiredload profile on projecting rod 554. Initially, when projecting rod 554is inserted into ferule 560, the internal diameter of ferule 560 isgreater than the diameter of projecting rod 554. Ferule 560, withprojecting rod 554 inserted, is pushed into conical hole 562. The coneangle of conical hole 562 matches the cone angle of the external surfaceof ferule 560. When ferule 560 with inserted projecting rod 554 ispushed into conical hole 562, ferule 560 is pressed and collapses ontoproximal end 554 a of projecting rod 554. Continued pushing into conicalhole 562 continues to press ferule 560 onto projecting rod 554 until adesired pressure is attained, forming constricted neck 554 b onprojecting rod 554. For example, if the outer diameter of a fiberglassprojecting rod 554 is about 12 mm, then the diameter of constricted neck554 b may be reduced by about 0.1 mm.

Use of ferule 560 may apply circularly uniform pressure on the outersurface of projecting rod 554, thus avoiding mechanical failure of outerfibers of projecting rod 554 which could result in reduced pullstrength. The ferule inner profile, relating the inner diameter offerule 560 to its insertion distance, and the desired pressure may becalculated by taking into account the effects of Poisson's effectcontraction on the rod diameter resulting from exertion of the high pullforce. The desired pressure profile on projecting rod 554 may becalculated to minimize the combined (von Mises) stress on projecting rod554. For example, at constricted neck 554 b of projecting rod 554, pullforces may be very high. Thus, at constricted neck 554 b, pressureforces should be sufficiently low to avoid high von Mises stress. Thepressure profile should increase gradually in accordance with thedecrease in the pull force due to the friction forces on the outersurface of projecting rod 554.

A brush-like flywheel construction may be advantageous. For example,such a construction is unlikely to catastrophically fail. If one ofprojecting rods 554 were to fail or disintegrate, the centrifugal forceswould throw the resulting debris to be thrown outward toward the wallsof a vacuum enclosure that encloses the rotor. Since brush-like flywheel550 typically includes hundreds of projecting rods 554, the kineticenergy of a single failed rod is relatively low. Therefore, arequirement for reinforced housing or for some other safety features maybe reduced or eliminated. In addition, safety factors that are used indetermining operating parameters may be relaxed somewhat relative toother types of flywheels. A flywheel system incorporating brush-likeflywheel 550 may be equipped with imbalance detectors that could senseany imbalance caused by a detached projecting rod 554. Upon detection ofsuch a failure, braking may be applied to brush-like flywheel 550, orbrush-like flywheel 550 may otherwise be brought to a gradual halt.Furthermore, since brush-like flywheel 550 is constructed primarily outof glass and metal, components may be recyclable and no use of anyhazardous material is required.

In accordance with some embodiments of the present invention, a weightmay be added to a distal end of each projecting rod of a brush-likeflywheel. Addition of such weights increases the moment of inertia ofthe flywheel.

FIG. 8 schematically illustrates a variant of the brush-like flywheelshown in FIG. 5A, in which the radially projecting rods are weighted.

Weighted brush-like flywheel 570 includes projecting rods 554 with endweights 572 added to the distal ends of the rods. Each end weight 572may be connected to a projecting rod 554 using one or more of theattachment techniques discussed above for attaching projecting rods 554to core 551 or to rod holders 552, or using another attachmenttechnique. End weights 572 may be constructed out of a dense material,e.g., steel or another material.

Attachment of end weights 572 to the distal ends of projecting rods 554may be advantageous. For example, the increase in moment of inertia mayincrease the quantity of energy that may be stored for a given angularvelocity without increasing the length of each projecting rod 554.Alternatively, attachment of end weights 572 may enable shortening eachprojecting rod 554, thus decreasing the lateral dimensions of weightedbrush-like flywheel 570 relative to a brush-like flywheel without endweights. Alternatively, weighted brush-like flywheel 570 may be spun ata slower speed than a brush-like flywheel without end weights to store asimilar quantity of energy.

In accordance with an embodiment of the present invention, a brush-likeflywheel may include projecting glass fibers that act as projectingrods. The flywheel may be constructed out of stacked assemblies, eachherein referred to as a cassette.

FIG. 9 schematically illustrates structure of a cassette of a glassfiber brush-like flywheel, in accordance with an embodiment of thepresent invention.

A plurality of flywheel cassettes 580 may be stacked to form a singlebrush-like flywheel with projecting rods in the form of projectingfibers. For example, the fibers may include glass. When stacked, theflywheel cassettes 580 are all centered about and mounted to a commoncentral shaft 588.

Flywheel cassette 580 includes two plates 582 (only one plate is shown)sandwiching columns 584. For example, the plates 582 may be constructedout of a dense material, such as steel or another material.

Columns 584 extend from one plate 582 to the other. Columns 584 arearranged in a circular pattern that is concentric with plate 582 andwith the axis of the flywheel. Columns 584 may be attached to plates 582using screws, or using another suitable attachment mechanism ortechnique.

Glass fibers making up fiber bundles 586 are each partially (or fully)wrapped about each column 584. For example, fiber bundles 586 may eachinclude a plurality of extremely thin fiber glass fibers, e.g., with atypical diameter of 7 microns to 20 microns. The fibers extendsymmetrically and by an equally amount (e.g., by a typical distance ofabout 15 cm) from either side of column 584.

Fibers of a fiber bundle 586 may be glued together at contact region 586a where the fiber bundle 586 bends around a column 584.

When the flywheel rotates, centrifugal forces cause fibers of each fiberbundle 586 to extend radially outward from the axis of the flywheel. Thecentrifugal forces act approximately equally on both extending ends offiber bundle 586. Thus, the effect of the centrifugal forces essentiallytends to hold fiber bundles 586 in place. A fiber bundle 586 thuswrapped around a column 584 may be advantageous of an arrangement whereglue or another holding method is depended upon to withstand or overcomethe centrifugal forces. Use of techniques whereby projections, such asfibers or rods, pass through the core, although balancing thecentrifugal forces, are limited as to the attainable density of theprojections.

Since the tensile strength of very thin fiber glass fibers is typicallyvery high (e.g., much higher that the tensile strength of ordinary glassrods), a flywheel including a plurality of cassettes 580 may be rotatedat very high speeds without reaching the maximal tensile limit of thefibers.

Use of projecting fiber bundles 586 may be advantageous. For example,the likelihood of catastrophic failure of the flywheel is reduced. Sincethe flywheel may include millions of individual fibers, the kineticenergy of each fiber is relatively very low.

FIG. 10 illustrates a technique to form a fiber bundle for the cassetteshown in FIG. 9. A standard filament winding machine (not shown) may beapplied to wind fibers 590 around two columns 584. Columns 584 may beheld in place by a suitable fixture or holder (not shown). For example,the distance between the columns 584 may be typically equal toapproximately 30 cm. Fibers 590 may be made of fiberglass, steel musicwire, or any other high tensile-strength fiber. After fibers 590 arewound around the two columns 584, fibers 590 are glued to form bundlesin the region where fibers 590 are wrapped around columns 584. After theglue cures, fibers 590 are cut, typically along midline 592 betweencolumns 584. Thus, two U-shaped bundles are formed (about each ofcolumns 584).

Alternatively or in addition, fibers 590 may be soaked in a matrixmaterial prior to winding. After winding, fibers 590 may be cut andformed into bundles (e.g., each bundle having a typical diameter ofabout 12 cm). The bundles may then be cured (e.g., thermally or at roomtemperature, depending on the matrix material).

In accordance with embodiments of the present invention, a rotor of themotor/generator unit is mechanically coupled to a shaft of the flywheelunit within a vacuum enclosure. The stator of the motor/generator unitis located outside the vacuum enclosure. Such an arrangement is hereinreferred to as an open-frame motor/generator unit.

FIG. 11 schematically illustrates an open-frame motor/generator unit, inaccordance with an embodiment of the present invention.

Open-frame flywheel energy storage system 600 includes flywheel 601enclosed within vacuum enclosure 150. Flywheel 601 may include abrush-like flywheel as shown, or another configuration of a flywheel.

Rotor 604 of open-frame motor/generator unit 602 is mechanicallyattached, in a cantilevered manner (with no bearing or other support ofrotor 604 other than shaft 142), to shaft 142 of flywheel 601. Rotor 604is housed within cap 162 of vacuum enclosure 150. (Cap 162 may be in theform of a curved dome.) Stator 606 of open-frame motor/generator unit602 is located outside of cap 160. Cap 162 is made of a magneticallysusceptible material (e.g., a glass composite material such as Kevlar®or fiberglass, or another material) so as to enable magnet couplingbetween rotor 604 and stator 606. For example, rotor 604 and stator 606may be separated by a typical distance of 3 mm, or another distance.

Stator 606 may be connected to an inverter of a high-voltage (HV) directcurrent (DC) bus, or other suitable circuitry.

Use of open-frame motor/generator unit 602 in a flywheel energy storagesystem may be advantageous. For example, enclosing rotor 604 withinvacuum enclosure 150 may minimize atmospheric drag on rotor 604.Bearings 120 that support flywheel 601 also support rotor 604, thusavoiding the cost of additional bearings. Stator 606 may be removed fromthe remainder of open-frame flywheel energy storage system 600 when nocharging or discharging is taking place. Such removal may reduce eddycurrent losses caused by rotor 604 rotating within the stator 606.Stator 606, being located outside of vacuum enclosure 150, may be cooledby natural convection or by forced convection (e.g., by a blower, fan,or pump). No electrical connections are required between componentswithin vacuum enclosure 150 and circuitry outside of vacuum enclosure150. A single stator unit may be shared with a plurality of flywheelunits by an automatic movement system.

In accordance with some embodiments of the present invention, aplurality of individual flywheel units may be coupled to one another toform a flywheel cluster. Each flywheel cluster may be coupled to asingle motor/generator unit. Each individual flywheel unit may bedesigned for a particular maximum rotation velocity. Coupling theflywheel units together may enable increasing the quantity of energythat is stored, without requiring modification of the design (e.g.,flywheel or bearings) of the flywheel units. Furthermore, the energy maybe stored or extracted without the cost of additional motor/generatorunits.

FIG. 12 schematically illustrates a flywheel cluster, in accordance withan embodiment of the present invention.

Flywheel cluster 1000 includes a stack of flywheel units 90. Eachflywheel unit 90 includes a flywheel 1002 (which may be a brush-likeflywheel, as shown, or another type of flywheel) that is individuallysupported and secured by a set of bearings 120 within its vacuumenclosure 150. Each flywheel unit 90 is permanently coupled to anadjacent flywheel unit 90 of flywheel cluster 1000 by magnetic coupling1004. Thus, all flywheel units 90 of flywheel cluster 1000 rotate intandem at a single rotational velocity.

A flywheel unit 90 a at one end of flywheel cluster 1000 is coupled tomotor/generator unit 204. For example, motor/generator unit 204 mayinclude an open-frame motor/generator unit as shown. As another example,flywheel 1002 of flywheel unit 90 a may be magnetically coupled tomotor/generator unit 204. As another example, motor/generator unit 200may be mechanically coupled to flywheel unit 90 a and enclosed withinvacuum enclosure 150 of flywheel unit 90 a.

In accordance with embodiments of the present invention, a flywheel unitmay include an active magnetic bearing. For example, use of activemagnetic bearings may enable long-life and reliable support of theflywheel in the vacuum environment, where use of air bearings isprecluded. Use of active magnetic bearings typically includes use ofanother set of conventional bearings (“landing bearings”) duringtransportation and initial activation of the flywheel unit. The activemagnetic bearings need to be powered at all times. Any interruption ofsupplied power would necessitate use of the landing bearings.

A typical active magnetic bearing system includes a shaft positionsensor (e.g., based on eddy current or capacitive sensors) to monitorthe position of the rotating shaft that is being stabilized. A rotorspins with the shaft (may be mounted to the shaft or may be identicalwith the shaft). Inductive actuators are used to attract the rotor orshaft being stabilized. Control and drive circuitry controls operationof the inductors in accordance with the sensed position of the shaft.

In accordance with embodiments of the present invention, a flywheel orother rotating load may be supported by mechanical bearings. Astabilization system may be provided to balance the rotating load. Forexample, the stabilization system may include an active magneticbalancing system.

FIG. 13A schematically illustrates an active magnetic balancing systemfor a flywheel of a flywheel energy storage system, in accordance withan embodiment of the present invention. FIG. 13B shows a top view of theactive magnetic balancing system shown in FIG. 13A.

Flywheel active magnetic balancing system 900 is configured to stabilizeshaft 940 of flywheel 920. Although flywheel active magnetic balancingsystem 900 is described herein as applied to a flywheel system forenergy storage, flywheel active magnetic balancing system 900 may beapplied to stabilize any rotating load whose shaft is supported bymechanical bearings.

Shaft 940 is continuously held in place by ball bearing assembly 910.For example, ball bearing assembly 910 may include a metal ball bearing,a ceramic ball bearing, a hybrid ball bearing assembly, or anothermechanical bearing. Flywheel 920 may be balanced to a high degree (e.g.,as required by ISO 1940 class G1). However, radial forces on ballbearing assembly 910 may limit the bearing's lifetime and may createsevere vibrations during operation. The radial forces may contribute tointernal friction and may cause overheating of ball bearing assembly910, reducing operational efficiency of a flywheel unit that includesflywheel 920.

Flywheel active magnetic balancing system 900 includes one or moresensors 950. Sensors 950 may include vibration or force sensors. Sensors950 are mounted on the stationary side of ball bearing assembly 910, orin close proximity to it. Each sensor 950 may give a fast and accuratereading of imbalance forces operating on the ball bearing assembly 910in a particular direction. Sensors 950 may be located sufficiently farfrom inductive actuators 960 to prevent inductive actuators 960 frominfluencing readings by sensors 950.

A magnetic stabilization assembly that includes a plurality of (e.g.,three) electromagnets is controllable to create a resultant magneticfield that reduces imbalance forces acting on shaft 940. Eachelectromagnet is included in an inductive actuator 960. Inductiveactuators 960 are mounted on a stationary (non-rotating) structure.Inductive actuators 960 may be operated to attract rotor ring 970. Rotorring 970 is mounted on shaft 940, or may be incorporated into or may beidentical with shaft 940. For example, rotor ring 940 may be made fromstacked layers of electrical steel (e.g., such as is used in transformercores). The use of thin silicone steel (e.g., of 0.2 mm thickness) cancontribute to reduced eddy current losses.

Control unit 980 may include a processor or other control circuitry. Forexample, control unit 980 may include an application-specific integratedcircuit (ASIC). Control unit 980 may be configured to receive a signalthat is indicative of vibration or force from sensors 950. An algorithmmay be applied to the signals to calculate how inductive actuators 960are to be driven in order to minimize or reduce the sensed forces thatact on ball bearing assembly 910.

Use of flywheel active magnetic balancing system 900 may beadvantageous. For example, stress and losses by ball bearing assembly910 may be reduced significantly, thus increasing reliability, servicelife, and time between maintenance for ball bearing assembly 910.Operation of flywheel 920 is not solely dependent on operation of themagnetic bearings, since temporary failure of the magnetic bearingswould enable continued operation while only temporarily increase theload on ball bearing assembly 910. Geometrical stability of the shaft(which may be a major concern with conventional magnetic bearings) isensured by ball bearing assembly 910. Changes in flywheel balance,either dynamically (as the rotation speed changes) or over time (due tocreep), may be compensated continuously. Control unit 980 may beconfigured to provide information about any sensed imbalance or creationof vibrations. Such provided information may be utilized to avoidcatastrophic failure events.

Control of flywheel active magnetic balancing system 900 may differ fromcontrol of a magnetic bearing system. In a typical magnetic bearingsystem, shaft position is measured and corrected. However, in flywheelactive magnetic balancing system 900, shaft 940 is fixed in space byball bearing assembly 910 and the magnetic field generated by inductiveactuators 960 exerts a force on rotor ring 970 (and on shaft 940). Ballbearing assembly 910 provides sufficient stiffness such that exertedforces do not cause significant movement or deflection of shaft 940.Thus, sensors 950, which include vibration or acceleration sensors, areused in flywheel active magnetic balancing system 900. Sensor 950measures vibration caused by imbalance of shaft 940. This vibration maybe described by a sinusoidal functional shape, where the phase of thesinusoidal function is determined by the position of sensor 950 relativeto a vector that describes the rotational imbalance. When flywheelactive magnetic balancing system 900 is not functional, imbalance forcesare countered by ball bearing assembly 910. When flywheel activemagnetic balancing system 900 is functional, the magnetic field exertsforces to counter the imbalance (and the radial forces on ball bearingassembly 910 are minimized). It may be noted that the vibrations thatare measured by sensor 950 are in principal the same whether the radialload is handled by ball bearing assembly 910 or by flywheel activemagnetic balancing system 900.

Control of flywheel active magnetic balancing system 900 may includecontrolling inductive actuators 960 to create a rotating force vectorwith the same rotation speed as that of shaft 940 (synchronous forcevector). For example, the driving frequency of the coil excitation (incase of three inductive actuators 960 placed at 120 degree intervals)may be half of the rotational speed of shaft 940. (For example, theforce exerted by each of inductive actuators 960 may be proportional tothe square of the current in that inductive actuator 960. Thus, when thecurrent is described by a sine wave, the frequency of the exerted forceis double that of the current.) The phase angle is varied over 360degrees while the vibrations are measured by sensor 950. A maximumsensed vibration is indicative that the magnetic force vector is inphase with the mechanical imbalance vector. The phase of the rotatingmagnetic force vector is then changed by 180 degrees (opposite themechanical imbalance) while its amplitude is varied from zero to apreset maximal value. When the amplitude is zero, all the imbalance ishandled by ball bearing assembly 910 alone. As the amplitude of themagnetic force vector is increased, the force on ball bearing assembly910 is reduced. When the force on ball bearing assembly 910 reversesdirection, the optimal amplitude is indicated. This procedure may berepeated from time to time to enable compensation for changes inflywheel balance.

Alternatively or in addition, the phase to be applied may be calculatedusing measurements from two sensors 950 sensing vibrations mounted witha 90 degree separation about the flywheel axis. A balancing calculationalgorithm that is executed by controller 980 may be configured todetermine desired phase angle for the balancing signal.

FIG. 13C schematically illustrates control of the active magneticbalancing system shown in FIG. 13A using a three-phase H-bridge.

Inductive actuators 960 are driven by a three-phase H-bridge 981(similar H-bridges used in motor controllers). Three-phase H-bridge 981is driven by controller 982. Controller 982 is configured to providestandard three-phase control of inductive actuators 960. Controller 982is additionally configured to modify the phase of the three sine wavesignals that drive inductive actuators 960 so as to modify the phase ofthe rotating resultant force vector. Thus, the phase may be controlledto minimize vibrations or flywheel rotor imbalance as detected by sensor950.

Controller 982 is furthermore configured to compensate for geometricvariation of mechanical components due to various tolerances in theproduction and assembly of the inductive actuators 960 and rotor ring970. Such manufacturing tolerances (e.g., with typical magnitudes ofabout 0.1 mm) could cause variations in the resultant force (which isdependent on the reciprocal of the square of the distance betweeninductive actuator 960 and rotor ring 970). Thus, even when high-endmachining and wire cutting techniques are used, tolerances of ±0.1 mm to±0.01 mm could be present. Since the distance between rotor ring 970 andmagnetic actuators 960 may be about 0.3 mm, such tolerances could leadto considerable variance in the force and could require compensation,even if the actuators themselves and the driving current were to beperfectly accurate.

Lookup table (LUT) 983 may include corrections to the drive signals thatare based on geometrical reference data 984. Geometrical reference data984 may be measured during production may be and can be utilized bycontroller 982, together with LUT 983 and with rotation data provided byencoder 952, to facilitate calculation of the correction. Thus, rotatingforce vector may be correctly generated constantly to compensate forvariation in dimensions due to manufacturing and assembly tolerances.

FIG. 13D schematically illustrates control of the active magneticbalancing system shown in FIG. 13A using power amplifiers.

Inductive actuators 960 are fed by power amplifiers (POAMP) 986. Eachpower amplifier 986 may be individually controlled. Thus, each phase maybe individually controlled. LUT 983 may be utilized in controlling poweramplifiers 986 so as to compensate for geometrical tolerances providedby the geometrical reference data 984. More than three inductiveactuators may be individually controlled, enabling additionalflexibility in correcting individual drive signals and increasing theaccuracy of the rotating force vector.

In accordance with an embodiment of the present invention, a flywheelrotor may include an automatic balancing system. Wireless control may beutilized to change a configuration of an eccentric structure on theflywheel shaft in order to adjust the balancing of the flywheel (withoutuse of slip ring or other contact-based communication between acontroller and balancing mechanism).

FIG. 14A is a schematic illustration of a flywheel balancing assemblyfor a flywheel energy storage system, in accordance with an embodimentof the present invention. FIG. 14B is a side view of the flywheelbalancing assembly shown in FIG. 14A.

Flywheel balancing assembly 1200 may be mounted on a flywheel rotor.Flywheel balancing assembly 1200 may be controlled by controller 1240.Controller 1240 may be stationary (not rotating). Controller 1240 maycontrol operation of flywheel balancing assembly 1200 in accordance withsensed signals from imbalance sensors.

Flywheel balancing assembly 1200 includes two worm gears 1210. Each wormgear 1210 includes eccentric borehole 1215 that is aligned parallel tothe axis of worm gear 1210. Eccentric borehole 1215 causes worm gear1210 to have an eccentric mass distribution that may be adjusted tobalance the flywheel by rotation about that longitudinal axis of wormgear 1210.

Each worm gear 1210 may be rotated by rotation of a worm screw 1220 byoperation of motor assembly 1230. Worm screw 1220 and motor assembly1230 also rotate together with the flywheel rotor. (Placement of twoworm screws 220 and two motor assemblies 1230 in a symmetric mannerabout the flywheel rotor may avoid introducing imbalance in theflywheel.) Each motor assembly 1230 may be controlled by a signalgenerated by controller 1240 to rotate clockwise or counterclockwise bya controlled rotation angle. When no command signal is generated bycontroller 1240, worm gears 1210 self-lock and there is no movement ofworm gears 1210.

A control signal may be transmitted wirelessly from a stationarycontroller 1240 to a motor assembly 1230 that rotates together with theflywheel. For example, an optical signal may be generated by controller1240, e.g., by a light emitting diode (LED), diode laser, or otherdevice. The optical signal may be detected by a photo-sensor (e.g.,photovoltaic cell) that is mounted on the rotating components offlywheel balancing assembly 1200. Transmitted commands may bedistinguished from one another by transmitter location (e.g., angular orradial encoding), by wavelength of the optical signal, or by anothercharacteristic of the optical signal.

An electromagnetic signal may be generated by controller 1240. A motorassembly 1230 may be provided with a inductor to enable inductivepowering of motor assembly 1230.

In accordance with embodiments of the present invention, a flywheelenergy storage system may include a plurality of flywheel units arrangedin one or more arrays (e.g., as shown in FIG. 2). The flywheel energystorage system may be controlled to determine a rate at which amotor/generator unit is storing energy in a flywheel unit or isextracting energy from the flywheel unit. The controlling may be suchthat a rate of energy storage or extraction is kept a substantiallyconstant level, even when a motor/generator unit is disconnected fromall flywheel assemblies (e.g., is being moved from one flywheel assemblyto another). The controlling may be coordinated with a smart grid thatis configured to determine a level of power that is to be provided by,or to be stored in, the flywheel energy storage system.

FIG. 15 schematically illustrates a flywheel energy storage system thatincludes an array of flywheel units, in accordance with an embodiment ofthe present invention.

Flywheel array energy storage system 303 includes a plurality offlywheel units 103 arranged in a plurality of flywheel groups 313 (e.g.,four flywheel groups 313). Each flywheel group 313 is provided with amotor/generator unit 200 and an associated power controller 326. Themotor/generator unit 200 and power controller 326 of a flywheel group310 is configured to couple with any flywheel unit 103 of that flywheelgroup 310. Power controller 326 is configured to control its associatedmotor/generator unit 200 store or provide electrical power at adetermined rate. For example, an electrical power storage or supply ratemay range from 0.5 kW to 15 kW (or another range). Motor/generator units200 are connected to central inverter 330. Central inverter 330 isconnected to alternating current (AC) mains power grid 390. Duringtypical operation, each motor/generator unit 200 may operate at atypical power level. For example, at a typical power level of about 10kW per motor/generator unit 200, a flywheel array energy storage system303 with four flywheel groups 313 may operate at a total power level ofabout 40 kW.

The power level of each motor/generator unit 200 and of central inverter320 is controlled by local controller 340. Local controller 340 isconfigured to operation of the entire flywheel array energy storagesystem 303.

Local controller 340 may be in communication with one or more remotecontrollers 360. Communication with remote controller 360 may take placevia smart grid network 350, or via another network or communicationschannel.

Alternatively or in addition, local controller 340 may be incommunication with site controller 370. Site controller 370 may beconfigured to manage energy flow at a renewable energy generation siteor another energy generation and storage facility. (Site controller 370may manage multiple local controllers 340. At a small site, localcontroller 340 may function as a site controller.)

Movement control system 316 may be operated to move a motor/generatorunit 200 from one flywheel unit 103 to another. During the movement, thepower to or from the flywheel group 313 with which that motor/generatorunit 200 is associated may be interrupted. Local controller 340 may beconfigured to reduce or minimize effects of the interruption.

Local controller 340 may be configured to operate so as to reduce oreliminate effects of the interruption in one flywheel group 313. Forexample, prior to and during movement, the power level of other flywheelgroups 313 of flywheel array energy storage system 303 may be graduallyincreased while the power level of that flywheel group 313 is graduallydecreased to zero.

For example, in a flywheel array energy storage system 303 with fourflywheel groups 313 may have a nominal power level of 10 kW per flywheelgroup 313. Prior to movement, power of the flywheel group 313 in whichthe movement is to take place may be reduced to 5 kW. Concurrently, thepower levels of two other flywheel groups 313 are increased to acompensating power level of 12.5 kW. Next, the power level of theflywheel group 313 in which movement takes place is reduced to zero,while the power of the other three flywheel groups 313 is increased to13 kW, 14 kW and 14 kW, respectively. The modified power levels aremaintained until the motor/generator unit 200 of that flywheel group 313is coupled to another (or the same) flywheel unit 103. At this point,the power level may be gradually changed until the power levels of allflywheel groups 313 are restored to their original nominal levels (e.g.,10 kW).

Local controller 340 may be configured to ensure that only onemotor/generator unit 200 of only one flywheel group 313 of flywheelarray energy storage system 303 is being moved (or being prepared to bemoved) a any given time. For example, local controller 340 may beconfigured (e.g., by programmed instructions) to enable limitedflexibility with regard to the overall energy storage limits of flywheelarray energy storage system 303. For example, a flexibility margin(e.g., of about 1 kWh) would enable one motor/generator unit 200 toremain connected to a particular flywheel unit 103 until anothermovement (and coupling) of another motor/generator unit 200 is complete.Any variation in power level of flywheel array energy storage system 303may be communicated (e.g., via smart grid network 350 or otherwise) toremote controller 360 or to site controller 370.

Local controller 340 may be configured to change power levels offlywheel groups 313 so as to enable disengaging a motor/generator unit200 from a flywheel unit 103 so as to reduce the operating hours of thatmotor/generator unit 200.

Typically, a flywheel array energy storage system is configured tooperate at high DC voltage (e.g., as high as 1000 V, or more typicallyat about 400 V). For example, a typical solar cell array may include aDC bus that is created by aggregating the output of several few solarpanels in order to create a high-voltage and high-current bus. The busis connected to an inverter that converts the output to synchronizedalternating current (AC) voltage that is fed to an AC mains power grid.In some cases, an active power management unit may be used in order tooptimize power transfer from each solar array to the central bus.

Typically, and as described above, a flywheel energy storage system mayinterface to the AC mains power grid. The flywheel energy storage systemmay store energy from the grid, or supply stored energy to the grid. Themultiple energy conversions involved may affect the efficiency of theprocess.

In accordance with an embodiment of the present invention, a flywheelenergy storage system may be directly connected to a renewable energydirect current (DC) bus. In this manner, the number of power conversionsmay be reduced.

FIG. 16 schematically illustrates a flywheel energy storage system thatis directly connected to a renewable energy DC bus, in accordance withan embodiment of the present invention.

Renewable energy flywheel energy storage system 400 is designed to storeenergy from renewable energy generating devices. Renewable energygenerating devices, such as wind turbine 402 or solar cell array 404,are connected to high voltage DC bus 435. High voltage DC bus 435 mayoperate at a variety of possible DC voltages. For example, the DCvoltage of high voltage DC bus 435 may range from 400V to 1000 V. The DCvoltage may be fed into main inverter 450. Main inverter 450 isconfigured to convert DC voltage to an AC one- or three-phase voltagethat is synchronized to phase and frequency of AC mains power grid 390.

In some cases, wind turbine controller 411 and solar cell controller 421may be in communication with system controller 460. Wind turbinecontroller 411 and solar cell controller 421 may be configured tomonitor operation wind turbine 402 and solar cell array 404,respectively. For example, wind turbine controller 411 and solar cellcontroller 421 may report to system controller 460 the current supplylevel at which energy is supplied by wind turbine 402 or solar cellarray 404. Wind turbine controller 411 and solar cell controller 421 mayreport any detected malfunction of wind turbine 402 or solar cell array404 that could reduce power that is supplied or that is forecasted to besupplied.

According to an embodiment of the present invention, high voltage DC bus435 is connected to energy routing unit 440. Energy routing unit 440 isconfigured to function as a managed energy router. Energy routing unit440 may direct power from renewable sources to flywheel storage DC bus431. The directed power is fed into inverter units 432 which drive theDC motors of motor/generator units 200. Alternatively, energy routingunit 440 may direct energy supplied by inverter units 432 via flywheelstorage DC bus 431 to be fed to main inverter 450. Alternatively, energyrouting unit 440 may direct power from high voltage DC bus 435 to maininverter 450.

System controller 460 may be configured to control energy routing unit440 in accordance with a programmed decision system. Alternatively or inaddition, system controller 460 may be configured to issue commandsbased on information received from other entities or remote controllers,e.g., via the smart grid network 350 or via other viable means ofcommunications.

When the voltage of flywheel storage DC bus 431 is essentially the sameas that of high voltage DC bus 435, then flywheel storage DC bus 431 maybe connected directly to high voltage DC bus 435 without energy routingunit 440. Flow of energy to and from a flywheel unit may be managed byinverter 432 and flow of energy into main inverter 450 may be managed byan inverter controller. In this manner, the simplification of the systemmay avoid the cost and losses due to additional components, and systemreliability and resilience may be improved.

Use of renewable energy flywheel energy storage system 400 may beadvantageous. Overall efficiency may be improved by avoiding multiplepower conversions (since DC current is converted to AC current only whenbeing fed into AC mains power grid 390). Energy routing unit 440 mayincrease flexibility of the system by enabling increasing power to maininverter 450 when there is a temporary decrease in the power supplied bywind turbine 402 or by solar cell array 404. Energy routing unit 440 mayroute excess energy to be stored by the flywheel storage system. Forexample, regulations may limit the electrical power that is fed into ACmains power grid 390. Thus, surplus energy is utilizable at a latertime. Connection of all units to a single high voltage DC bus 435 and acentral system controller 460 may enable individual control of eachflywheel unit.

In accordance with an embodiment of the present invention, a flywheelenergy storage system may include a constant voltage DC bus. A constantvoltage DC bus may avoid use of a master-slave configuration, where afailure or fault in the master unit could result in failure of theentire system to operate.

FIG. 17 schematically illustrates a flywheel energy storage system thatincludes a constant voltage DC bus, in accordance with an embodiment ofthe present invention.

Constant DC voltage flywheel energy storage system 1300 includes aplurality of flywheel units 1310. Each flywheel unit 1310 is associatedwith motor/generator unit 1320 and controller/controller/inverter unit1330.

For example, an associated flywheel unit 1310 and motor/generator unit1320 may include an open frame flywheel/rotor within a vacuum enclosure,and a outside of the vacuum enclosure, as shown. Other configurations offlywheel units and motor/generator units may be used (e.g., amotor/generator unit whose rotor is magnetically coupled to a flywheelunit within a vacuum enclosure, or a motor/generator unit that ismechanically coupled to a flywheel unit and that is also enclosed withinthe vacuum enclosure).

Components of constant DC voltage flywheel energy storage system 1300may be controlled a local system controller 1370 via control bus 1350.Alternatively or in addition, local system controller 1370 may becontrolled by global controller 1390 via network 1380, which controlsComponents of constant DC voltage flywheel energy storage system 1300via control bus 1350.

Each controller/inverter unit 1330 is connected to high voltage DC bus1340. High voltage DC bus 1340 may connect to AC mains power grid 390via rectifier/inverter 1360. In some cases, high voltage DC bus 1340 maybe connected to one or more renewable energy sources via renewableenergy power bus 1345. The DC voltage of high voltage DC bus 1340 may bekept constant. For example, the DC voltage of high voltage DC bus 1340may have a value of approximately 650 V, or another value.

During a charge phase of operation, controller/inverter unit 1330operates in a charge mode to convert DC current for high voltage DC bus1340 to an AC voltage. For example, a DC bus voltage of 650 V may beconverted to an AC voltage with amplitude of about 300 V for a flywheelrotational velocity of 20,000 rpm, or having an amplitude of about of600 V for a flywheel rotational velocity of 40,000 rpm. The AC voltageis fed to stator coils of motor/generator unit 1320 to create a torqueto accelerate the flywheel of flywheel unit 1310.

During a discharge phase of operation, an AC voltage describable by asine wave is induced within the stator coils of motor/generator unit1320 operating in a discharge mode. The amplitude of the induced ACvoltage is of similar magnitude to the amplitude of the AC voltage thatis fed into the stator coils during the charge phase (e.g., 300 V with20,000 rpm and 600 V with 40,000 rpm). Controller/inverter unit 1330includes bridge circuit and pulse-width modulation (PWM) controller1335. For example, bridge circuit and PWM controller 1335 may include anH-bridge circuit and a PWM control circuit. Controller/inverter unit1330 utilizes the inductance of motor/generator unit 1320 to step up thevoltage signal for feeding into high voltage DC bus 1340 in acurrent-limited mode. The current-limited mode causes the voltage of theoutput of controller/inverter unit 1330 to increase to a voltage levelthat enables current flow from controller/inverter unit 1330 to highvoltage DC bus 1340 at a predetermined current level. With suchcurrent-limited operation, multiple inverter units 342 may concurrentlyfeed power into a single high voltage DC bus 1340 without mutualinterference. Local system controller 1370 may control a set point ofcurrent to be fed by controller/inverter unit 1330 to DC bus 1340. Thus,the rate of discharge of energy of a flywheel of each flywheel unit 1310is controlled by local system controller 1370.

A constant DC voltage flywheel energy storage system 1300 that includesa constant voltage DC bus may be advantageous. For example, charging anddischarging of multiple flywheel units 1310 (with the correspondingcontroller/inverter units 1330 operating in charge or discharge mode,respectively) that are connected to a single high voltage DC bus 1340may be performed concurrently. A single controller/inverter unit 1330supports both charging and discharging functions. Direct connection ofhigh voltage DC bus 1340 to renewable energy power bus 1345 is enabled.

Constant DC voltage flywheel energy storage system 1300 may enableincreased functional flexibility and resilience. For example, failure ofan individual controller/inverter unit 1330 (except due to a shortcircuit) would not affect proper operation of the inverter units 1330.In contrast, a system configured for sequential operation, e.g., asystem in which different flywheel units would provide power atdifferent voltages, would limit power levels to that of an individualflywheel unit 1310.

1. A flywheel energy storage system for storing electrical energy, thesystem comprising: a plurality of flywheels; a plurality ofmotor/generator units, each motor/generator unit of the plurality ofmotor/generator units coupled to a flywheel of the plurality offlywheels; a plurality of controller/inverters, each controller/inverterbeing electrically coupled to a motor/generator unit of the plurality ofmotor/generator units; a direct current (DC) bus electrically coupled tothe plurality of controller/inverters and to a main inverter; and acentral controller to control each controller/inverter of the pluralityof controller/inverters so as to set a discharge rate for each of theflywheels when its motor/generator unit is operating in a dischargemode, and to increase a voltage level of a voltage signal generated bythat motor/generator unit when operating in the discharge mode and fedinto the DC bus.
 2. The system of claim 1, wherein a controller/inverterof said plurality of controller/inverter comprises a bridge circuit anda pulse-width modulation controller.
 3. The system of claim 2, whereinthe pulse-width modulation controller comprises an H-bridge circuit anda pulse-width modulation circuit.
 4. The system of claim 1, wherein thecentral controller is configured to control a controller/inverter ofsaid plurality of controller/inverters to operate in a discharge modewhile concurrently controlling another controller/inverter of saidplurality of controller/inverters to operate in a charge mode.
 5. Thesystem of claim 1, wherein the main inverter is coupled to the DC busand to an alternating current power grid.
 6. The system of claim 1,wherein the DC bus comprises a constant voltage DC bus.
 7. The system ofclaim 1, wherein the DC bus is a high voltage DC bus operating at avoltage of at least 400V.
 8. The system of claim 7, wherein an operatingvoltage of the high voltage DC bus is between 400V and 1000 V.
 9. Thesystem of claim 8, wherein the source controller is in communicationwith the central controller.